The present invention relates to a system for exhaust gas purification and a method for exhaust gas purification, both used for effectively purifying harmful substances present in an exhaust gas, particularly hydrocarbons (HC) and the like generated in large amounts during the cold start of an engine.
In the FTP test (LA-4 mode) conducted in connection with the intensified exhaust gas regulation of the United States, 70 to 80% of the total HC emission amount is emitted within 140 seconds (within the first hill of Bag 1) during the cold start of an engine after engine cranking. During this period, however, considerable time passes until a conventional three-way catalyst reaches its operational temperature, and most of the HC is therefore exhausted without being purified.
In order to solve this problem, techniques were proposed in which an electrically-heated catalyst (hereinafter referred to as an EHC) and/or an adsorbent comprising zeolite as the main active component, is disposed in an exhaust pipe.
For example, Japanese Patent Application Laid-Open No. 31359/1993 discloses a catalytic converter for car exhaust gas purification in which three elements of an EHC, an adsorbent and a main catalyst are arranged in series in any desired order in an exhaust gas flow path.
SAE Paper No. 920847 proposes a by-pass system in which a valve is provided at the upstream side of an adsorbent so as to desorb the HC adsorbed on the adsorbent, after a catalyst has been sufficiently activated.
However, in Japanese Patent Application Laid-Open No. 31359/1993, no mention is made on the optimum arrangement of the above-mentioned three elements for the decrease of emissions during cold start and, moreover, fails to consider reducing of the electric power to be applied to the EHC.
The technique employing such a by-pass system as mentioned in the SAE Paper No. 920847 is satisfactory in the purification performance, but it has many problems in practical application. For example, this system is intricate and the valve in the exhaust pipe has low heat-resistance.